The present invention relates to a reverse development method capable of reversing images in electrophotographic copying machines and electrostatic recording apparatus or the like.
Conventionally, as the methods of forming a positive image from a negative image, a method of using toner charged to the same polarity as that of an electrostatic image on a recording member and a method of changing the bias voltage to be applied to a development electrode are known. These methods are described in detail in by R. M. Schaffert "Electrophotography", The Focal Press, London and N.Y. 1965, As the development methods for conducting the reverse development, a liquid development method and a magnetic brush development method employing a two-component type developer comprising toner and carriers are known. As will be described in detail later, these methods have been employed in practice without causing significant problems. However, in the case of the two-component type developer which is a mixture of toner and carriers, toner is consumed as the developer is used, so that the toner concentration becomes lowered, and a developed image with a sufficient density is not obtained when the developer is used continuously. Therefore, it is necessary to keep the quantity of toner in the developer constant by use of a toner concentration detection apparatus which acts to maintain a predetermined toner concentration. Accordingly, the development apparatus employing the two-component type developer has shortcomings in that the apparatus is complex in mechanism and high in cost.
Recently an inexpensive copying apparatus which does not necessitate much maintenance has been demanded, so that a development method employing a one-component type developer consisting of a magnetic toner has come into use. In the case of this method, reverse development cannot be performed satisfactorily by use of the same reverse development method used with the two-component type developer.
Referring to FIGS. 1 and 2, the reverse development method using conventional magnetic brush development will now be explained. In these figures, on a photoconductor 1, there exist positive charges with a potential V.sub.I in the latent image area and positive changes with a potential V.sub.N in the non-latent image area. A development roller 2 is charged to the same polarity as that of the electrostatic latent image, and a bias voltage V.sub.B which is slightly lower than the potential of the latent image area is applied to the development roller 2. As the developer is used a two-component type developer consisting of carriers 3 comprising iron filings, and positive charged toner 4. During the copying process, the bias voltage V.sub.B is constantly applied to the development roller 2. In the magnetic brush development employing such two-component type developer, spacing between the photoconductor 1 and the development roller 2 is so great that the action of the development roller 2 as a counter electrode is weak. As shown in FIG. 1, the number of lines of electric force 5 directed to the photoconductor 1 from the development roller 2 is so small that even if the bias voltage V.sub.B is applied to the development roller 2 when development is not performed, much toner is not deposited on the photoconductor 1, so that the photoconductor 1 is not smeared by the toner.
On the other hand, in the case of a development method employing a one-component type non-magnetic developer consisting only of toner, spacing between the development roller 2 and the photoconductor 1 is as small as approximately 50 .mu.m, so that electric lines of force 6 are condensed on the photoconductor 1 as shown in FIG. 3. In the case where a magnetic toner is employed, spacing between the development roller 2 and the photoconductor 1 is slightly greater than in the case of the non-magnetic toner. However, since the resistivity of the magnetic toner is low, the electric lines of force generated by the development bias are apt to be directed to the photoconductor through the developer and accordingly, the counter electrode effect is greater than in the case where the two-component type developer is used. As a result, toner is deposited on the photoconductor by application of the bias voltage of the same polarity as that of the latent electrostatic image to the development roller during non-development as well as during development.
Referring to FIGS. 4 and 5, representative development apparatuses of the above-mentioned types will now be explained.
FIG. 4 is a schematic sectional view of a development apparatus employing a high resistivity non-magnetic toner. In FIG. 4, reference numeral 7 represents a development roller. The toner supplied from a hopper 38 is positively charged triboelectrically by a triboelectric charging blade 9. On a photoconductor 10, there is formed a negative electrostatic image with positive charge, and to the development roller 7, a bias voltage of the same polarity as that of the electrostatic image is applied, whereby reverse development is performed. At the time of development, positively charged toner is deposited in all areas except the electrostatic charge area on the photoconductor 10, so that a positive image is formed. However, if the bias voltage is continuously applied at the time of non-development, toner is deposited on the photoconductor 10 by the bias effect and consequently, the load of a cleaning apparatus (not shown) for cleaning the photoconductor 10 becomes great and the toner is wasted.
Referring to FIG. 5, there is shown a development apparatus employing a low resistivity magnetic toner. In FIG. 5, a development roller 11 comprises a non-magnetic sleeve 12 and a magnet 13 disposed inside the non-magnetic sleeve 12 to which a positive bias voltage is applied. On the sleeve 12, there is formed a magnetic brush by the magnetic toner supplied from the hopper 8. Since the magnetic toner is low in resistivity, it is not charged in advance, but has positive charge thereon due to the bias voltage applied to the sleeve 12, so that a positive image is formed with the toner deposited in an area without positive charged electrostatic image on the photoconductor 10. In this case, the magnetic toner is deposited on the photoconductor during non-development, causing the same shortcomings as in the above-mentioned example. In both cases, when the residual toner cannot be removed sufficiently with the load at the time of cleaning, a good image cannot be obtained due to the toner at the next reverse development. Particularly, in the process for obtaining one copy with two revolutions of a photoconductor, where development and cleaning are performed by the same apparatus, this problem is amplified.